This invention relates to microwave antenna shielding, and more particularly relates to an improved protective wall system both for protecting satellite earth station reception antennas from interfering external sources of radiation, and for minimizing the discharge of stray microwave radiation departing from the vicinity of a satellite earth station antenna.
Antennas used to communicate with satellites by transmitting or receiving radio frequency (RF) signals in the microwave bands often must be shielded to operate effectively. When receiving signals from a satellite, antennas are highly susceptible to interference, usually in the range of 3.7 to 4.2 GHz (3.7 to 4.2 billion Hertz), derived from nearby RF sources or ground reflection. Such interference may result from direct transmitting sources, such as other satellite microwave communication antennas, or from the microwave portions of a telephone communication network. RF interference may also consist of RF signals reflected from nearby ground objects. In radar antennas, reflection from ground sources is known as "clutter." When transmitting signals, especially in a location having a relatively dense concentration of satellite communication stations, the antenna must be shielded to minimize escape of RF signals at ground level that could interfere with other antennas. Such outgoing signals are usually in the range of 5.9 to 6.5 GHz.
Prior antenna shield systems have adopted several approaches to interference and clutter elimination. Becuase microwave radiation is propagated along line of sight paths, the most common protection technique has been to install antennas in a location surrounded by solid, microwave absorbing or reflecting obstructions. Natural barriers such as hills, valley walls or quarry walls are effective in minimizing interference; where natural barriers are unavailable, earthen barriers or reflective shield fences must be constructed.
In densely populated or highly urbanized areas, natural barriers are often either unavailable or inconveniently located. Use of earthen berms also requires construction of substantial and usually relatively tall structures, which are costly, and in any event require more land area than is often available in urban locations. Accordingly, the most common method for eliminating RF interference or clutter in urban areas has been to construct shield walls or fences.
In their simplest form, shield walls or fences usually consist of a simple electrically conducting fence surrounding at least the sides of the antenna that are susceptible to RF interference, unwanted clutter, or radiation discharge. Such fences have frequently consisted of a metallic mesh fence with sufficiently small openings to block an acceptable portion of the unwanted radiation. In all mesh fences or reflectors, the size of the opening is a direct function of the RF signal that the system is designed to reflect. Generally, shield fences must produce at least a 30 dB reduction (known as "attenuation") in the radiation passing through the shield to be effective.
Like all electromagnetic radiation, RF radiation is diffracted when passed by a sharp or knife-like edge of RF reflective material. Thus, while RF interference reflecting fences can be constructed to adequately stop substantial quantities of microwave radiation from transmission through the wall or fence, a simple conducting wall or screen is normally inadequate to fully eliminate all RF interference or clutter, because the wall or fence usually acts as a knife edge that diffracts RF radiation into the vicinity of the antenna.
To eliminate diffraction problems, a variety of techniques or constructions have been employed. For example, shield walls or fences have been constructed with a large diameter rounded top to reduce the similarity of the shield wall top to a kife edge. Alternatively, walls have been constructed to extreme heights above the protected antennas to allow difraction of RF signals only to positions above the normally tall dish antenna.
Other, more complicated diffraction control systems exist in the prior art. U.S. Pat. No. 3,982,249, issued to Toman, discloses a combined diffraction control edge and radiation screen designed to reduce undesired ground reflection effects or other low angle radiation problems in a microwave signal transmission system. Similarly, Becker & Millett, A Double-Slot Radar Fence for Increased Clutter Suppression, IEEE Transactions on Antennaas and Propagation, Vol. AP-16 No. 1, Jan. 1968, discloses a system of two thin slots positioned on end parallel to the top edge of a shield fence. Ruze, Radar Ground-Clutter Shields, IEEE Proceedings, Vol. 54, No. 9, Sept. 1966, discloses serrated edges on the top of a radiation shield or fence to disperse diffracted RF signals. These and other systems are at least partially effective in attenuating diffracted RF interference.
Prior art radiation walls or fence constructions, while effective, have usually been costly, or difficult to construct and adjust, or both. Most wall or fence constructions must be of substantial height to effectively block interference in the vicinity of a large dish antenna. Such large walls or fences usually require extensive excavation to support, and may require construction of a complicated support structure to carry the expanse fine mesh necessary to reflect a sufficient portion of RF interference. Additionally, prior constructions for control of diffracted RF interference have usually been difificult to construct, or, as in the case of serrated edges, difficult to properly "tune" or adjust when installed. Moreover, prior shield systems have typically required constructions that are entirely custom designed for each shielded location; few if any of the components are modular or usuable in more than one location.
Accordingly, it is an object of this invention to provide an improved construction for shielding an antenna against RF interference.
It is a further object of this invention to provide a construction for shielding an antenna that both protects against transmission of RF signals through the shield and controls diffraction of RF signals across the top of the shield.
Another object of this invention is to provide an improved microwave wall or shield construction that may be quickly and inexpensively installed.
A further object of this invention is to provide a microwave wall or shield that utilizes standardized or modular parts and requires minimum custom design.
Still another object of this invention is to provide an improved microwave wall or shield construction that allows relatively easy adjustment of the diffraction control structure.
Those and other objects of the invention are achieved by providing a wall structure adapted to minimize radio frequency wave interference in the vicinity of an antenna. The wall is composed of a plurality of modular wall panels, each having a layer of electrically conductive materials that substantially block or reflect transmission of electromagnetic microwave signals. The wall structure includes a top structure creating thin generally horizontal gap in the conductive material at the wall's top, that acts to control diffraction over the wall. In the preferred embodiment, the layer of electrically conductive material is a mesh of expanded metal embedded in modular concrete panels, and the gap comprises the spece between the top of the mesh and a pair of vertically adjustable metal strips mounted on the wall.
These and other features of the invention are apparent from a study of the drawing figures and the following detailed description of the preferred embodiment.